The present invention relates to a sheet post-processing apparatus that sets sheets carried out from an image forming apparatus such as a printer, into a bunch and then staples or folds the sheet bunch, as well as an image forming system comprising the sheet post-processing apparatus. More particularly, the present invention relates to improvements in a sheet stapling mechanism and a sheet folding mechanism.
In general, post-processing apparatuses which set sheets carried out from an image forming apparatus or the like and then staple and fold the sheets into a booklet or fold the sheets without performing a stapling operation are well known. Such a post-processing apparatus is used as, for example, a system apparatus for a bookbinding process or the like which is connected to a sheet discharging port in an image forming apparatus.
For example, Patent Document 1 [Japanese Patent Laid-Open No. 2006-008384] discloses an apparatus that collects sheets from an image forming apparatus in a conveying path, staples the resultant sheet bunch at a central part thereof, folds the sheet bunch at a fold line position corresponding to the center of the sheet bunch, and then houses the folded sheet bunch in a stacker. Patent Document 1 [Japanese Patent Laid-Open No. 2006-008384] discloses the apparatus having a stopper member that regulates the leading end of sheets so as to move the sheets collected in the path between a staple position and a fold position, the stopper member being configured to elevate and lower in the path. 
Patent Document 2 [Japanese Patent Laid-Open No. 2005-041660] discloses an apparatus configuration having a flapper member at a sheet carry-in port in order to prevent, when sheets are loaded and collected in a path, a succeeding sheet from advancing between preceding sheets to cause incorrect collating. The flapper member guides the succeeding sheet onto the collected sheets.
As is well known, with the above-described sheet folding device, when sheets are loaded and collected in a path on which a sheet folding mechanism is located, if the preceding sheets are carried into the path and the succeeding sheet is then carried in so as to follow the trailing end of the preceding sheets, the succeeding sheet may slip in between the loaded preceding sheets to cause incorrect collating. In particular, when the position of the leading end of the collected sheets is regulated so as to place the sheets at a predetermined fold position or a staple position, if the succeeding sheet has a length size different from that of the collected sheets, for example, the succeeding sheet is shorter than the collected sheets, a gap is created between the trailing end of the collected sheets and the trailing end of the succeeding sheet. This prevents the succeeding sheet from being guided onto the collected sheets, increasing the likelihood of out-of-order pages. The conventional art thus adopts a mechanism that collects sheets into a bunch at an upstream position different from the sheet fold position or staple position and then conveys the resultant sheet bunch to the staple position or the fold position.
Patent Document 2 [Japanese Patent Laid-Open No. 2005-041660] thus proposes the mechanism having the flapper member that maintains the correct order of sheets collected at the sheet carry-in port. Patent Document 2 [Japanese Patent Laid-Open No. 2005-041660] discloses the mechanism having the flapper member hanging from the sheet carry-in port to a collecting guide to guide the sheets. The  carried-in sheets are wholly fed in a conveying direction by a predetermined amount, switched back, and placed under the flapper member so that the flapper member can overlap the sheets to carry in the next sheet. That is, Patent Document 2 [Japanese Patent Laid-Open No. 2005-041660] proposes the mechanism having the flapper that guides the sheets to the sheet carry-in port while maintaining the correct order of the sheets, the mechanism using a sheet leading end regulating member to move each entire sheet in a forward-backward direction (up and down) to offset the trailing end of the sheet guided by the flapper member (see FIG. 9 of Patent Document 2 [Japanese Patent Laid-Open No. 2005-041660]).
However, when a guide mechanism such as the flapper and a sheet position regulating mechanism that adjusts the position of the trailing end of the collected sheets are arranged at the sheet carry-in port as in the case of Patent Document 2 [Japanese Patent Laid-Open No. 2005-041660], the apparatus disadvantageously becomes complicated. That is, every time a sheet from the sheet bunch collected on the collecting guide is carried in through the carry-in port, the sheets already collected into the bunch are offset from one another forward or backward in a conveying direction. This requires a complicated mechanism. With sheets consecutively delivered through the carry-in port, it is disadvantageous to offset the bunched sheets forward or backward in the conveying direction for every sheet carry-in operation, in terms of operation time, timing, and a control mechanism.
It is therefore an object of the invention to solve the problems stated above and provide a guide member at the sheet carry-in port so as to be able to offset the sheet to be delivered without the need to move the sheets on the collecting guide forward or backward. Further, the guide member at the sheet carry-in port is configured  to be elastically deformable or rotatable around a shaft. Further, pivotally moving the guide member so as to pass over the sheet bunch enables the trailing end of the sheet carried in along the guide to be offset so as to allow the next sheet to be carried in.
It is also an object of the present invention to provide an inexpensive sheet folding device of a simple structure which can prevent a possible sheet jam or possible out-of-order pages when sheets are collected at a post-processing position and which enables the resultant sheet bunch to be subsequently accurately placed at a staple position or a fold position.
Further objects and advantages of the invention will be apparent from the following description of the invention.